This invention relates generally to magneto-resistor devices employed in position and speed sensors, and more particularly, to a method and apparatus for electrically exciting a magneto-resistor sensing device to achieve increased sensitivity over a broad range of operating temperatures, while preventing damage due to self heating at elevated operating temperatures.
It is well recognized that magneto-resistor (MR) devices can be employed in sensors for measuring the position and/or speed of moving ferromagnetic objects (see for example, U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456, which are assigned to the assignee of the present invention). In such applications, an MR device is typically biased with a magnetic field and then electrically excited using some form of constant current or constant voltage source. As a ferromagnetic object, such as a toothed wheel or rack, is moved proximate and relative to the MR device, the flux density of the magnetic field passing through the MR device is modulated, which in turn varies the resistance of the MR device. The position and/or speed of movement of the ferromagnetic object is then generally determined by processing or conditioning the resulting modulation in the voltage drop that appears across the MR device.
Conventionally, problems are encountered when the above type of MR sensors are required to function over a broad range of operating temperatures. The resistance of an MR device exhibits a significant negative temperature coefficient, i.e., the resistance decreases with increasing operating temperature. As a result, when a constant current source is used to excite the MR device, the amplitude of the quiescent voltage drop produced across the MR device decreases with increasing temperature, which reduces the sensitivity of the MR sensor. Conversely, when a constant voltage source is used to excite one or more MR devices in a bridge configuration, such as described in U.S. Pat. No. 5,038,130 issued to Eck et al., the quiescent voltage drop appearing across any MR device must be set sufficiently low to prevent destruction due to self heating at higher operating temperatures as the MR resistance decreases.
Consequently, there exists a need for a method and apparatus for optimizing the electrical excitation of a magneto-resistor device that is adapted to sense variations in a magnetic field over a wide range of operating temperatures.